1. Field of the Invention
The present invention relates to a gas sensor and a method for fabricating the same, and more particularly, to a gas sensor having a built-in micro heater and a method for fabricating the same.
2. Discussion of the Related Art
In various applications, a gas sensor detects gas leakage, oxygen deficiency, and pollution materials made of nitrogen or carbon oxide. In addition, gas sensors control combustion conditions in engines and boilers.
A conventional semiconductor gas sensor, used in applications as described above, usually comprises a gas sensing portion, a heater portion, and an explosion-proof net. The operation of this gas sensor will now be explained.
First, when gas contacts a sensing layer, charge transfer occurs between gas molecules and the sensing layer, thereby changing the electrical conductivity of the sensing layer, i.e., the resistance. The electrical current passing through the electrodes is correspondingly changed. In this manner, the presence of gas is detected.
When a current flows through the heater, the gas sensing layer is heated by the heater. This improves the sensitivity and responsivity of the sensing layer, while also removing contaminants from the sensing layer.
According to a general method not employing semiconductor technology, the heater having the aforementioned performance is embodied in the form of coil of metal lines of Ni--Cr, Ta or Pt, as shown in FIG. 1. The coil-type heater is primarily used to heat ceramic tubes. However, the coil-type, heater is not available for integrated devices such as flat-type sensors.
In order to overcome the drawbacks of the coil-type heater, a thick film-type heater has been proposed which heats a ceramic substrate using a heater pattern formed in screen printing. The gas sensor having the thick film-type heater made with screen printing will now be explained with reference to FIGS. 1 to 3.
FIG. 1 is a plan view of a conventional thick film-type heater. FIG. 2 is a cross-sectional view of a conventional gas sensor having the thick film-type-heater shown in FIG. 1.
As shown in FIG. 1, a, screen in which a heater pattern 4 is formed on a ceramic substrate 1, and the screens are spaced at predetermined intervals. Then, printing is carried out using a paste of the heater material, such as Pt or RuO.sub.2 paste. Next, heat treatment of ceramic substrate 1 is performed at a high temperature, thereby removing organic materials in the paste. As a result, only the heater material of Pt or RuO.sub.2 remains on ceramic substrate 1 in the form of the heater pattern, thereby completing the heater.
As shown in FIG. 2, a conventional gas sensor consists of a ceramic substrate 1, an electrode 2 formed on the overall surface of the ceramic substrate 1, a sensing layer 3 formed on the overall surface of the electrode 2, and a thick film-type heater 4 formed on the back of the ceramic substrate 1.
However, in this conventional gas sensor, the thick film-type heater 4 formed by the screen printing method is too thick, i.e., greater than tens of micrometers, to fabricate fine patterns. In addition, the thick film-type heater increases the consumption of power, and is difficult to apply to semiconductor fabricating technology.
Meanwhile, wet etching and dry etching has also been used to form the heater pattern. The process of forming the heater pattern using etching will now be explained.
A heater material such as Pt or Au is deposited on a supporting layer, and the heater material formed on undesired portions is etched using photolithography, thereby finishing the heater pattern. However, the etching method as described above complicates the process because an etchant is used. The metal (such as Pt or Au) is difficult to etch, thereby preventing accurate etching.
Accordingly, a lift-off method has become widely used because a separate etch solution is not required in the heater pattern formation process, and a material difficult to etch can be selectively removed, for example, Pt or Au. The lift-off method will now be briefly described.
First, a photoresist is coated on a supporting layer, and the photoresist is selectively removed using photolithography to expose a portion of the supporting layer where a heater material is desired and to leave the photoresist on a portion of the supporting layer where a heater material is not desired.
Then, heater material is deposited on the supporting layer and photoresist, and the resulting structure is dipped in acetone to dissolve the photoresist using ultrasound. As a result, the photoresist and the heater-material deposited on the photoresist is removed, leaving the heater material only on the heater pattern where the supporting layer is exposed.
Recently, a method for fabricating a micro heater having a small structure through the aforementioned lift-off method using semiconductor technology has been proposed. This technology will now be briefly explained with reference to FIGS. 3a to 3d.
FIGS. 3a and 3d are cross-sectional views of the sequential manufacturing process of a conventional method for fabricating a heater of a gas sensor using the lift-off method.
As shown in FIG. 3a, a silicon oxide or silicon nitride is grown on a silicon substrate (not shown) to form a supporting layer 12. Photoresist 13 is coated on the supporting layer 12, and, as shown in FIG. 3b, and selectively etched leaving the photoresist only where a heater material, which will be deposited in the following step, will not be required. Through this process, a heater pattern 13a is formed. Here, the upper edge of the remaining photoresist is made with an overhang A. The purpose for the heater pattern 13a formed with the photoresist overhang A will be described below.
In the conventional manufacturing process of the gas sensor, photoresist is coated on the supporting layer, and then the photoresist is vertically etched to be left only on a portion where the heater material is not required, forming the heater pattern. The heater material is deposited on the overall surface of the heater pattern, and the heater pattern, which is left in the unrequired portion, is removed using the lift-off process. Here, the heater material comes into contact with the side of the heater pattern, whose side is not etched and is left without change during the photoresist etch process. Accordingly, the heater pattern of the unrequired portion is not perfectly removed.
Therefore, when the upper edge of the heater pattern has the overhang structure, the heater material does not make contact with the side of the heater pattern.
Then, as shown in FIG. 3c, a heater material 14 is deposited on the overall surface of the supporting layer 12, and the resulting structure is dipped in acetone and the heater pattern 13a is dissolved in the acetone using ultrasound. At this time, the heater material 14 on the heater pattern 13a is removed as the heater pattern 13a is removed. The heater material 14 is left only on the heater pattern portion where the supporting layer 12 is exposed, thereby completing the heater 14a.
However, as shown in FIG. 3b, even if the photoresist 13 is patterned in such a manner that its upper edge has overhang structure A, the heater material 14 is deposited on the supporting layer 12 by falling down on the heater pattern 13a not vertically but at various angles. As a result, the heater material is deposited even on the lower side of the overhang structure so that both ends of the heater material are higher than the center. Therefore, as shown in FIG. 3d, an undesired blade-shaped vertical pattern B is formed on the sides of the heater 14a.
If the semiconductor gas sensor is manufactured as the aforementioned heater, an insulation layer formed between the heater and electrode is so thin that the blade-shaped vertical pattern formed on both sides of the heater, may undesirably come into contact with the electrode. Accordingly, insulation between the heater and electrode is destroyed, thereby deteriorating the characteristics of the gas sensor.